Operation Manual – QoSH3C S5500-EI Series Ethernet Switches Chapter 1 QoS Overview1-2The new services have one thing in common: they all have special requirements fordelivery performances such as bandwidth, delay, and delay jitter. For example, videoconferencing and VOD require the guarantee of high bandwidth, low delay and lowdelay jitter. Some key services such as the transaction handling and the Telnet do notnecessarily require high bandwidth but they are highly dependent on low delay andneed to be processed preferentially in case of congestion.The emergence of new services brings forward higher requirements for the servicecapability of the IP network. In the delivery process, users hope to get better services,such as dedicated bandwidth for users, reduced packet loss rate, management andavoidance of network congestion, control of network traffic, provision of packet priority,and so on, instead of just having packets delivered to the destination. To meet theserequirements, the network service capability need to be further improved.1.4 Occurrence and Influence of Congestion and theCountermeasuresQoS issues that traditional networks face are mainly caused by congestion. Congestionmeans reduced service rate and extra delay introduced because of relativelyinsufficient resource provisioned.1.4.1 Occurrence of CongestionCongestion is very common in a complicated environment of packet switching onInternet. The diagram below gives two examples:Figure 1-1 Traffic congestion1) Packets enter a switch over a high-speed link and are forwarded out over alow-speed link.2) Packets enter a switch through multiple interfaces of the same rate at the sametime and are forwarded out on an interface of the same rate.If the outbound traffic exceeds the line rate, the traffic encounters the bottleneck ofresources and congestion occurs.Besides bandwidth bottleneck, any insufficiency of resources for packet forwarding,such as insufficiency of assignable processor time, buffer size, and memory resourcescan cause congestion. In addition, congestion will also occur if the traffic that arrives
